A long range goal of this laboratory is to understand how the enzymic behaviors of vascular endothelial and smooth muscle cells interact to maintain both arterial structural integrity and normal vascular tone. The specific objective of the work proposed is to determine how the activity of the plasma membrane 5'-nucleotidase (5-ribonucleotide phosphohydrolase, E.C. 3.1.3.5) is regulated in these two cell types. The kinetic properties of the enzyme in cultured arterial smooth muscle cells, in cultured arterial endothelial cells, and in freshly collected endothelial cells will be determined. The ratio of synthesis and degradation of enzyme protein will be measured in cultured smooth muscle cells, which maintain in vivo enzyme levels. Endothelial cells in culture lose much of their activity; this loss will be characterized, and a search will be made for factors which promote 5'-nucleotidase activity in cultured endothelial cells.